In LTE, like in other wireless communication systems, a mobile terminal may need to contact the network (e.g. via an eNodeB) without having a dedicated resource in the Uplink (from terminal to base station). To handle this, a random access procedure may be available, according to which a terminal or UE that does not have a dedicated UL resource may transmit a signal to the base station. The first message of this procedure is typically transmitted on a special resource reserved for random access (which is not normally dedicated to a specific terminal, but available to all terminals intending to perform random access), a physical random access channel (PRACH). This channel can for instance be limited in time and/or frequency (as in LTE), see e.g. FIG. 1.
The resources available for PRACH transmission are provided or indicated to the terminals as part of the broadcasted system information in system information block 2 (SIB-2) (or as part of dedicated RRC signaling in case of e.g. handover).
The resources consist of a preamble sequence and a time/frequency resource. In each cell, there are 64 preamble sequences available. Two subsets of the 64 sequences are defined, where the set of sequences in each subset is signaled as part of the system information. When performing a (contention-based) random-access attempt, the terminal selects at random one sequence in one of the subsets. As long as no other terminal is performing a random-access attempt using the same sequence at the same time instant, no collisions will occur and the attempt will, with a high likelihood, be detected by the eNodeB.
When the terminal is to be connected via multiple cells or cell groups, e.g. in dual connectivity, activation of different cells or cell groups (e.g., via random access) may be performed or fail without the nodes not directly associated to the respective group being informed. This may lead to problems in managing or controlling terminals in dual connectivity.